tools: Add iommu subfolder

To store tools and libraries that help testing the IOMMU, create a folder
in tools/iommu. These can be for example userspace clients that control
specialized DMA engines, or libraries that exercise mm features using DMA.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Change-Id: I4b0b3e3cfc5a92d10b7f9464c2d626431c720576